The lecithin:cholesterol acyl transfer (LCAT) reaction produces cholesteryl esters and lysolecithin in plasma. The rate of LCAT is related to the plasma lipoprotein concentrations. During pregnancy there are pronounced elevations of the lipid and lipoprotein concentrations. Therefore, we studied the LCAT rate and its relation to the lipid levels in plasma lipoproteins in 19 healthy women before conception, every sixth to eighth week during pregnancy, and 8 weeks after delivery. In the first part of gestation the mean molar LCAT rate (the amount of cholesteryl esters produced during a certain time, in micromoles per liter per hour) remained unchanged, whereas pronounced elevations were seen in the very low-density lipoprotein (VLDL), high-density lipoprotein (HDL), and HDL2 levels. The molar LCAT rate did not increase until the last trimester of pregnancy, when it reached a maximal 20% mean increase simultaneous with the maximal increase of the mean triglyceride and VLDL levels and a slight decline of the HDL2 elevation. The mean fractional LCAT rate (the part of unesterified cholesterol that is esterified during a certain time, in percent per hour) showed a continuous decrease from the fourteenth until the twenty-eighth week, simultaneous with a progressive rise of the mean cholesterol and low-density lipoprotein (LDL) concentrations. During pregnancy the molar LCAT rate was positively correlated to the VLDL concentration and negatively to the HDL2 level, and the fractional LCAT rate was negatively correlated to the LDL concentration.